The present invention relates to microphones and, more particularly, to a new differential!microphone having improved frequency response and sensitivity characteristics.
The most common approach to constructing a directional microphone is provided by an apparatus comprising sound inlet ports defined by juxtaposed tubes that communicate with a diaphragm. The two sides of the microphone diaphragm receive sound from the two inlet ports. The sound pressure driving the rear of the diaphragm travels through a resistive material that provides a time delay. The dissipative, resistive material must be designed to create a proper time delay in order for the net pressure to have the desired directivity.
It is important that the net pressure on the directional microphone is proportional to the frequency of the sound, and thus has a 6 dB per octave slope. The net pressure is: also diminished in proportion to the distance between the ports. Reducing the overall size of the diaphragm results in a proportional loss of sensitivity. It can be observed that the 6 dB per octave slope and the dependence on the distance dimension remain even in microphones devoid of the resistive material. A microphone without the resistive material is normally called a differential microphone or a pressure gradient microphone.
Directional microphones, which are commonly used in hearing aids, are normally designed to operate below the resonant frequency of the diaphragm. This causes the response to have roughly the same frequency dependence as the net pressure. As a result, the microphone output is proportional to frequency, as is the net pressure.
The uncompensated directional output exhibits a 6 dB per octave high pass filter shape. To correct for this frequency response characteristic, a 6 dB per octave low pass filter is incorporated in the hearing aid device, along with a gain stage. This yields a xe2x80x9cflatxe2x80x9d response. The microphone package incorporates a switch to allow the user to select between the two response curves.
The problem of electronically compensating for the 6 dB per octave slope of the diaphragm response is that it causes a substantial degradation in noise performance. Any thermal noise introduced by the microphone itself, along with the noise created by the buffer amplifier, is amplified by the gain stage in the compensation circuit. The significant increase in noise is very undesirable.
Hearing aid manufacturers have found it necessary to incorporate switches on hearing aids that allow users to switch to a non-directional microphone mode in quiet environments, where the directional microphone noise proves most objectionable.
The noise inherent in conventional, directional microphones has caused hearing aid microphone designers to use a relatively large port spacing of approximately 12 mm. This is considered to be the largest port spacing that can be used while still achieving directional response at 5 kHz, the highest frequency for speech signals.
Creating small directional microphones is dependent upon the product of frequency and port spacing. The distance factor indicates that sensitivity of the device is reduced as its overall size is reduced.
Traditionally, compensating the output signal to achieve a flat frequency response has been traditionally accomplished electronically. This has lead to the amplification of noise sources.
The present invention seeks a new approach to solving the aforementioned problems. It has been discovered that the mechanical structure employed in the directionally sensitive ears of the fly, Ormia ochracea, can act as a model for a hearing aid microphone having sound sensitivity without drastic amounts of frequency compensation. A diaphragm patterned after the Ormia ochracea ears is very well suited to silicon microfabrication technology.
The current invention provides a directional microphone having a one micron thick silicon membrane with dimensions of approximately 1 mmxc3x972 mm. The directional microphone has improved sensitivity, a reduced noise level, and a frequency response that is comparable to existing high performance miniature microphones.
In accordance with the present invention, there is provided an improved directional microphone or acoustic sensor having greater sensitivity and reduced noise. The directional microphone or acoustic sensor comprises a rigid, one micron thick polysilicon membrane having dimensions of approximately 1 mmxc3x972 mm. The membrane is supported upon its central axis by beams having torsional and transverse stiffness. The total damped area of the microphone is between approximately 1.5 and 2.5xc3x9710xe2x88x926 m2. The distance between centers of the two sides of the device is approximately 10xe2x88x923 m. The resonant frequency in the rotational mode is in a range of between approximately 700 to 1,000 Hz, and the resonant frequency of the translational mode is in the range of between approximately 40,000 and 45,000 Hz. The total mass of the device is between approximately 2.0 and 3.0xc3x9710xe2x88x928 kg. The mass moment of inertia about an axis through the supports is in a range of between approximately 9.0 and 10xc3x9710xe2x88x9215 kgm2. The damping constant is in a range of between approximately 9.5 and 10xc3x9710xe2x88x925 N-s/m, and is designed to provide critical damping. The signals from the microphone are filter compensated to achieve a flat frequency response over a range, typically between the 250 and 8,000 Hz octave bands.
It is an object of this invention to provide an improved acoustic device.
It is another object of the invention to provide a directional microphone or acoustic sensor of new design, having higher sensitivity and lower noise than do conventional directional microphones.
It is an additional object of the invention to provide a directional microphone which may be fabricated using silicon microfabrication techniques.